In many regions of the country, a high percentage of homes have outdoor swimming pools. Pools must be sanitized to prevent growth and spread of bacteria, viruses, algae and insect larvae that can cause disease. Swimming pool water must contain low levels of bacteria and viruses to prevent the spread of diseases and pathogens between users. This is typically done by using filters, ensuring that the pools have adequate flow rate and chemical disinfectants, such as chlorine, bromine or mineral sanitizers. Pumps and mechanical filters are often used to filter pathogens out of the water and chemical disinfectants, such as bromine, are used to make the water inhospitable to pathogens.
There are three main types of swimming pool filters, namely, a DE (Diatomaceous Earth) type filter; and a cartridge type filter and a sand type filter.
DE filters use diatomaceous earth to filter particles out of the water. The filters are composed of plastic grids inside of a plastic type of fabric. DE powder coats the grids and filters out tiny debris. If the pressure rises in the filter, the system automatically backwashes, similar to a sand filter, recharging itself with more DE powder. DE filters can cause some inefficiency and water flow loss because they tend to run at a higher pressure than cartridge filters.
Cartridge filters are considered to be the most economically low in maintenance. Water runs through a filter inside this particular system, catching any debris. It is very similar to water filtering systems that are used in homes. These type of pool filters do not clog up as much as the other pool filters, and two types of filter elements can be used in the cartridge filter system. The less expensive elements may be cheaper to replace, but they also need replaced more frequently. The more expensive elements tend to last longer. Cartridge filters are created to run on lower pressure. They filter out more particles than sand but not as many as DE filters. These filters can be cleaned once or twice during the swimming season by simply hosing them off with water.
The most inexpensive of the three kinds of pool filters are the sand filters. This type of filter works by filtering water through a bed of sand. Lateral tubes at the bottom of the sand then separate the clean water from the dirty water. Water is pushed through the filter sand, and as the filter separates the water, the dirty water is sent to the top while the clean water exits through the bottom. Debris can plug up and block sand filters which causes the pressure to increase in the filter and a drop in the flow of water. This can be fixed by running the system in reverse to backwash the filter, leaving it cleaned. The filter is then placed on its rinse mode where it repacks the sand back into the filter. Maintenance on a sand filter can be done manually every couple of weeks to keep the sand filter in proper working order. Sand filters are inexpensive and easy to replace. The downside to owning a sand filter is that it does not remove small particles that the DE and cartridge filters are capable of removing.
A conventional sand filter includes a vessel or tank that holds the sand and other internal parts and includes a multi port valve or control valve which permits a number of different functions to be selected and performed. For example, the following functions are commonly found on a standard multi port valve: (1) filter: normal filtering and vacuuming; (2) backwash: for cleaning the filter bed of accumulated debris; (3) rinse: use after completing the backwash cycle to ensure all dirty water is rinsed from the tank to waste; (4) waste: by-passes the tank for draining or lowering the pool water level and for vacuuming heavy debris directly to the waste line; (5) recirculate: water is recirculated through the valve by-passing the filter; and (6) closed: shuts off the flow from the pump to the filter. The internal parts of the filter include: (1) a top diffuser that allows the pool water to enter the tank and distributes the water inside the tank; (2) media (sand) that captures the dirt and debris from the pool water as it flows through; and (3) laterals that are located under the media and allows the pool water to exit the tank and flow back to the pool.
Relatively little is known about the capabilities of common swimming pool filters to remove waterborne pathogens. Recent research has found that swimming pool sand filters typically remove approximately 25% of Cryptosporidium oocysts. These levels of removal are inadequate to prevent outbreaks of cryptosporidiosis, which is supported by number of outbreaks detected and investigated each year by the Centers for Disease Control and Prevention (CDC).
Pool filters tend to be designed for keeping swimming pools looking clear and beautiful, which is not the same as removing all of the waterborne pathogens. The swimming pool industry has traditionally relied on disinfectants, such as chlorine, to control the spread of waterborne diseases. The drinking water industry did largely the same thing until chlorine-resistant pathogens forced changes in the 1980's (for Giardia) and the 1990's and beyond (for Cryptosporidium). The chlorine-resistant pathogens forced the drinking water industry to put considerable emphasis on filtration optimization to achieve physical removal of these pathogens. US drinking water regulations are continuing to become more and more stringent on pathogen removal in order to safeguard public health. The swimming pool industry could consider taking the same approach.
What is desired is an improved swimming pool sand filter that offers the ease and low cost of a conventional sand filter and further offers improved removal of pathogens, including Cryptosporidium. 